1. Field of the Invention
The present invention relates to an optical recording medium and an optical recording-reproducing method for reproducing information or recording-reproducing information by irradiation with a light beam, particularly, to a technology for improving recording density.
2. Description of the Related Art
Optical disc memories for reproducing information or for recording-reproducing information by irradiation with a light beam have been widely put to practical use for recording, reproducing and storing audio signals, video signals and computer data as a recording apparatus having high capacity, capable of high-speed access, and portability, and such optical disc memories are expected to be further improved in the future for high recording density (high recording capacity) and high-speed operation.
The recording density of an optical disc can be increased by various approaches including, for example, use of a mastering laser beam having a shorter wavelength, use of a light source mounted in the apparatus, i.e., a semiconductor laser in general, having a shorter wavelength, increase in the numerical aperture of an objective lens, hereinafter referred to as the NA, decrease in the thickness of a substrate (or a cover layer) on the light incident side, mark length recording, land-groove recording and improvements in modulation-demodulation system.
In addition to the approaches exemplified above, proposed is super-resolution reproduction technology utilizing recording medium films that produces a high effect of increasing the recording density. The super-resolution reproduction technology was proposed initially with respect to the magneto-optical disc (MOD). In the super-resolution reproduction technology, a reproducing layer is magnetically coupled with the recording layer by means of exchange coupling or magnetostatic coupling, in which an optical aperture smaller than a reproduction spot is formed in the reproducing layer by utilizing heating achieved by irradiation with reproducing light and change in temperature caused by the exchange force or the magnetostatic force between the layers. The above super-resolution reproduction technology, which utilizes in principle the magnetic interaction between the layers, was considered to be high-density technology inherent in MOD.
An attempt to form a super-resolution reproducing layer, in which an optical constant is non-linearly changed upon irradiation with reproducing light, on the light incident side of the recording layer was reported later in respect of a read-only disc (ROM). Since it is possible in principle to apply the method to not only MODs and ROMs but also to phase-change recording discs (PCD) and write-once discs (R or WO) typically having a dye recording layer, various proposals have been made to date in respect of the super-resolution reproduction medium using a nonlinear optical film. The technology, called super-RENS (super-resolution near-field structure), belongs to the super-resolution medium technology.
In the conventional super-resolution reproduction technology using a nonlinear optical film, proposed are a heat mode nonlinear optical film represented by Sb, Sb2Te3, Te, Ge—Sb—Te, or AgOx and a photon mode nonlinear optical film represented by a photochromic film or a semiconductor fine particle dispersed film.
All of the conventional super-resolution reproduction technologies using these nonlinear optical films utilize an optical response of the nonlinear optical film alone. In the conventional super-resolution reproduction technology, the nonlinear optical film was non-linearly changed continuously during irradiation with the reproducing light so as to perform the reproducing operation while reducing the reproducing spot size. In the conventional super-resolution reproduction technology, however, load applied to the nonlinear optical film is high, making it difficult to obtain a practical number of repetitions for reproduction.